In the first of a series of papers on the iodine(V) oxide system, the chemical and physical properties associated with iodine(V) oxide in its anhydride (I2O5) and hydrated states (HI3O8 and HIO3) are examined. The three forms of the oxide have been investigated utilizing differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and powder X‐ray diffraction (PXRD). Furthermore, the hydration rates governing the conversion of the anhydride (I2O5) to the initial hydrate (HI3O8) and later to the final hydrated state (HIO3) are reported and discussed. Results from this study suggest that the hydration mechanism for I2O5→HI3O8 begins with an accelerating period described as a nucleation and growth phase followed by a decelerating period that is diffusion limited. The initial rate of hydration was observed to be governed by a nucleation and growth mechanism, which was inhibited by covering the surface of the particle with an inert metal. Based on this investigation the initial rate of hydration appears to be strongly dependent on the anhydride’s available surface area which facilitates nucleation and growth of HI3O8. The final step, HI3O8→HIO3, proceeds through an initial induction period followed by a continuous acceleratory period unlike the first hydration step.